High-purity metal chromium and ferrochromium low in impurities are used for electronic materials and corrosion-resistant and heat-resistant super alloys. In the present invention, these metal chromium and ferrochromium are generically called “chromium-containing metals”. An economically available ore serving as a chromium source of the chromium-containing metal is chromite (FeO.Cr2O3). Because it contains much iron, however, the upper limit of the Cr content in ferrochromium obtained from chromite is about 72 mass %. It is therefore the general practice to use chromium oxide (Cr203) available by refining chromite as a raw material for metal chromium.
The known manufacturing methods of metal chromium include the aluminum-thermit process of reducing powdery chromium oxide with powdery metal aluminum, as disclosed in Japanese Unexamined Patent Application Publication No. 60-36632, the silicon reduction process of melting chromium oxide in an arc furnace and reducing the same with metal silicon, as disclosed as a manufacturing method of high-purity chromium-iron alloy in Japanese Examined Patent Publication No. 58-7700, and the electrolytic reduction process of electrolytically reducing a chromate solution and causing precipitation of metal chromium on the cathode in Japanese Unexamined Patent Application Publication No. 62-47436. These known manufacturing methods have respective advantages and disadvantages.
In the aluminum-thermit process, for example, it is possible to manufacture metal chromium easily in a simple equipment. This is however based on a batch process, resulting in a small throughput per batch, a low production efficiency, and the process uses expensive metal aluminum as a reducing agent, thus leading to a high manufacturing cost. Aluminum serving as a reducing agent remains in the manufactured metal chromium. Furthermore, the strong reducing atmosphere causes reduction of refractory constituents used for furnace lining, and the reduced constituents are entangled into the metal chromium, posing problems in purity.
In the silica reduction process, metal silicon serving as a reducing agent is lower in price than metal aluminum, and can reduce oxygen with a stoichiometrically smaller amount than aluminum. It is therefore possible to manufacture metal chromium at a lower cost than in the aluminum-thermit process even by taking into account the consumption for heating chromium oxide. In addition, continuous production in an arc furnace is also possible at a high production efficiency. The silicon reduction process is more advantageous than the aluminum-thermit process also in this term. It has however a serious quality problem in that silicon serving as a reducing agent remains in an amount of about 0.7 mass % in the manufactured metal chromium.
In the aluminum-thermit process and the silicon reduction process, if metal chromium is manufactured in a state of insufficient reduction by using a small amount of metal aluminum or metal silicon serving as a reducing agent, the amount of aluminum or silicon remaining in metal chromium decreases. However, this leads to a deteriorated reduction yield of chromium oxide and an increase in the amount of oxygen in manufactured metal chromium, thus causing another problem in manufacturing cost and in quality.
The electrolytic reduction process permits manufacture of relatively high-quality metal chromium. Because of the use of Cr2(SO4)3 as an electrolyte, however, manufactured metal chromium contains sulfur as much as 0.02 to 0.03 mass %. Further because this is aqueous solution electrolysis, the manufactured metal chromium contains from 0.3 to 1 mass % oxygen, and from 0.02 to 0.05 mass % nitrogen. Necessity of many treatments for refining the chromate solution results in economic problems such as complicated manufacturing steps, a high equipment cost, and a large power consumption.
A ferrochromium having a chromium content of at least 72 mass % cannot be manufactured in a single run alone of reduction-refining of chromite as described above. It is therefore the common practice to deiron the once manufactured ferrochromium through an acid treatment or the like, as is disclosed in Japanese Unexamined Patent Application Publication No. 6-4897. However, the conventional deironing treatment, including this method, cannot be considered to be high in production efficiency because of complicated treatment process.
Requirements for a chromium-containing metal as typically represented by metal chromium used in electronic materials and corrosion-resistant and heat-resistant superalloys are achievement of a higher purity and reduction of manufacturing cost. Regarding these requirements, the chromium-containing metals manufactured by the conventional methods mentioned above contain much impurity elements, and requires a higher manufacturing cost according as the purity is higher, and cannot be considered to satisfy these requirements.